Our previous studies provided evidence that guinea pigs that are vitamin C deficient and those that have been fasted, but supplemented with vitamin C, are equivalent with respect to the mechanisms responsible for decreased collagen and proteoglycan synthesis. Sera from both groups contain a circulating factor that inhibits these functions and DNA synthesis in cultured connective tissue cells and inhibition is reversed by insulin- like growth factor (IGF)-I. The presence of the inhibitor in sera was associated with an increase in low molecular weight IGF binding proteins (IGFBPs) that can inhibit binding of IGF-I to its cellular receptor. We have identified the binding proteins as IGFBP-1 and IGFBP-2. Antibodies to purified rat IGFBPs l and 2 were used to remove the corresponding guinea pig IGFBPs from fasted and scorbutic guinea pig sera. Removal of the IGFBPs also removed the ability of the sera to inhibit IGF-I-dependent functions such as DNA and collagen synthesis. These results unequivocally establish the identity of the inhibitor as IGFBPs. Taken together with our results showing that the IGFBPs and inhibitor are induced in parallel and that induction occurs prior to or concomitant with the decrease in collagen mRNA concentrations in bone, skin and cartilage, our studies suggest that the IGFBPs also function as inhibitors of collagen synthesis during fasting and vitamin C deficiency in vivo.